Gold nano-particles in a shape of a rod (gold nanorods) with uniform configuration have a strong absorption band in a region extending from visible light to near infrared rays, and it is possible to change its absorption peak positions easily by controlling configuration thereof. Gold nanorods have high aptitude as near-infrared probes because modification of their surface enables change of their physical properties.
As methods for manufacturing gold nanorods, an electrolytic method, a chemical reduction method and a photo-reduction method are conventionally known. With the electrolytic method (reference is made to Non-Patent Reference 1: Y. Y. Yu, S. S. Chang, C. L. Lee and C. R. C. Wang, J. Phys. Chem. B, 101, 6661 (1997)), a solution containing a cationic surfactant is electrolyzed by constant current, and gold clusters are leached from a gold plate at the anode, thereby generating gold nanorods. For the above-mentioned surfactant, a quaternary ammonium salt having a structure in which four hydrophobic substituents are bonded to a nitrogen atom is used.
In addition, a compound in which an autonomous molecular assembly is not formed, for example, tetradodecylammonium bromide (TDAB), is added. In the case of manufacturing the gold nanorods, the source of the gold supply is gold clusters that are leached from a gold plate at the anode, and gold salt, such as chlorauric acid, is not used. Ultrasonic waves are radiated during electrolysis, a silver plate is immersed in the solution, and the growth of the gold nanorods is accelerated.
The electrolytic method is characterized by the fact that the change of the area of the silver plate to be immersed separately from an electrode enables control of the length of the rod to be generated. The adjustment of the rod length enables setting of the absorption band in the near-infrared region from the vicinity of 700 nm to the vicinity of 1,200 nm. If the reaction condition is uniformly maintained, gold nanorods with a uniform configuration can be manufactured to an extent. However, the surfactant solution used for the electrolysis is a complex system containing excessive quaternary ammonium salt, cyclohexane and acetone, and because of indefinite elements, such as ultrasound wave radiation, it is difficult to theoretically analyze a cause-effect relationship between the configuration of the gold nanorods to be generated and various manufacturing conditions, and to optimize the manufacturing conditions for the gold nanorods. Furthermore, because of the nature of the electrolysis, it is not easy to scale up, making it unsuitable for the large-scale manufacture of gold nanorods.
With the chemical reduction method (reference is made to Non-Patent Reference 2: N. R. Jana, L. Gearheart and C. J. Murphy, J. Phys. Chem. B, 105, 4065 (2001)), NaBH4 reduces chlorauric acid and gold nano-particles are generated. Considering these gold nano-particles as “seed particles” and growing them in the solution results in obtaining the gold nanorods. The length of the gold nanorods to be generated is determined according to the quantitative ratio of the “seed particles” to the chlorauric acid added to the growth solution. With the chemical reduction method, it is possible to generate longer gold nanorods in comparison with the above-described electrolytic method. A gold nanorod having an absorption peak in the near-infrared region over 1,200 nm is reported.
As described above, in the chemical reduction method, two reaction baths for the preparation and reaction to grow the “seed particles” are required. Furthermore, although the generation of the “seed particles” is completed in several minutes, it is difficult to increase the concentration of the gold nanorods generated, and the generation concentration of the gold nanorods is one-tenth or less in comparison with that when using the electrolytic method.
With the photo-reduction method (reference is made to Non-Patent Reference 3: F. Kim, J. H. Song and P. Yang, J. Am. Chem. Soc., 124, 14316 (2002)), chlorauric acid is added to substantially the same solution as that in the electrolytic method, and ultraviolet irradiation results in the reduction of the chlorauric acid. For irradiation, a low-pressure mercury lamp is used. In the photo-reduction method, gold nanorods can be generated without producing seed particles. It is possible to control the length of the gold nanorods by the irradiation time. This method is characterized by excellently uniform configuration of the gold nanorods generated. With the electrolytic method, because a large quantity of spherical particles coexist after reaction, it is necessary to separate the spherical particles by centrifuging. However, with the photo-reduction method, since the ratio of the spherical particles is small, separation processing is unnecessary. Furthermore, there are advantages, for example, in that reproducibility is excellent, and gold nanorods of the same size can be almost certainly obtained using a standard operation.
In the meantime, the photo-reduction method requires 10 hours or more for the reaction. Furthermore, particles having an absorption peak at a position of over 800 nm cannot be obtained. In addition, there is the additional problem in that light from the low-pressure mercury lamp is harmful to the human body.
The present invention has resolved the above-mentioned problems in the manufacturing methods requiring several hours for the conventional manufacture of metal nanorods, and provides a method to enable the prompt and simple manufacturing of metal nanorods, such as of gold, silver or copper. Furthermore, the present invention provides a metal nanorod manufacturing method where the generation ratio of spherical metal nano-particles, intermixed as by-products, is reduced and no purification process after the reaction is required. In addition, the present invention provides a manufacturing method where the configuration control of the metal nanorods in a wide range enables control of the spectral characteristic in the region extending from visible light to near infrared rays. Furthermore, the present invention provides for the use of the manufactured metal nanorods.